The present disclosure relates to a motor-driven compressor in which a movable scroll is driven by an electric motor.
Japanese Laid-Open Patent Publication No. 2010-14108 describes an example of a motor-driven compressor that drives a movable scroll of a scroll type compressor with an electric motor. As shown in FIG. 7, a motor-driven compressor 70 (motor-driven scroll type compressor) of the above document includes a front housing 71 that accommodates a rotation shaft 72. FIG. 7 shows the motor-driven compressor 70 with its front end located at the right side and its rear end located at the left side. The rotation shaft 72 includes a front end, which is supported by a bearing 73a, and a rear end, which is supported by a bearing 73b. This allows the rotation shaft 72 to rotate. A shaft support 74 is arranged in the front housing 71. The compressor 70 includes a fixed scroll 75 and a movable scroll 76. The fixed scroll 75, the movable scroll 76, the shaft support 74, and the rotation shaft 72 are arranged in the compressor 70 from the rear toward the front in this order. A spiral wall 75a is formed in the fixed scroll 75, and a spiral wall 76a is formed in the movable scroll 76. The engagement of the spiral walls 75a and 76a forms a compression chamber 77 between the spiral walls 75a and 76a. 
A back pressure chamber 78, which is a back pressure region accommodating a rear end of the rotation shaft 72, is formed between the movable scroll 76 and the shaft support 74. A suction pressure region 79 is formed at the front of the shaft support 74 in the front housing 71. A discharge chamber 81 is formed between the fixed scroll 75 and a rear housing 80. The compression chamber 77 and the discharge chamber 81 are in communication with each other through a discharge port 82. An oil separation chamber 83 is formed in the rear housing 80. An oil separator 84, which separates lubrication oil from a refrigerant gas, is arranged in the oil separation chamber 83. The oil separation chamber 83 and the back pressure chamber 78 are in communication with each other through an oil supplying passage 85. The lubrication oil collected under a discharge pressure in the oil separation chamber 83 is supplied to the back pressure chamber 78 through the oil supplying passage 85.
An oil supplying bore 86 is formed in the rotation shaft 72. The lubrication oil in the back pressure chamber 78 is drawn through the oil supplying bore 86 into the suction pressure region 79, the pressure of which is lower than that of the back pressure chamber 78. The oil supplying bore 86 includes a first opening 86a, which opens toward the bearing 73a at the front end of the rotation shaft 72, a second opening 86b, which opens in the back pressure chamber 78 at the rear end of the rotation shaft 72, and a communication hole 86c, which communicates the first opening 86a and the second opening 86b. 
The refrigerant gas discharged into the discharge chamber 81 is drawn into the oil separation chamber 83 where the oil separator 84 separates lubrication oil from the refrigerant gas. The lubrication oil falls from the oil separator 84 and collects in the oil separation chamber 83. The lubrication oil collected in the oil separation chamber 83 is supplied to the back pressure chamber 78 through the oil supplying passage 85. The pressure of the lubrication oil supplied to the back pressure chamber 78 pushes the movable scroll 76 against the fixed scroll 75 and hermetically seals the compression chamber 77. The lubrication oil supplied to the back pressure chamber 78 also enters the oil supplying bore 86 through the second opening 86b and is drawn into the suction pressure region 79, the pressure of which is lower than the back pressure chamber 78. Here, the lubrication oil passes through the communication hole 86c and the first opening 86a, lubricates the bearing 73a, and returns to the suction pressure region 79.
However, in the motor-driven compressor 70 of Japanese Laid-Open Patent Publication No. 2010-14108, the lubrication oil supplied to the back pressure chamber 78 and entering the oil supplying bore 86 through the second opening 86b is always drawn to the suction pressure region 79. In other words, the back pressure chamber 78 and the suction pressure region 79 are always in communication with each other. This lowers the pressure of the back pressure chamber 78. As a result, the force pushing the movable scroll 76 against the fixed scroll 75 may become insufficient.